What do the subscripts mean in $N_{t,n,\sigma,L}$?
Here is a paper with the mathematical definition of each term:


  Let Nt,n,σ,L be all target functions that can be implemented using a
  neural network of depth t, size n, activation function σ, and when we
  restrict the input weights of each neuron to be |w|1 + |b| ≤ L.